Fragments Formed by McLafferty Rearrangements

Another photochemical reaction of ketones is dissociation to radicals that are familiar to us as simple cleavage products of these compounds (Equation 2.70). 

The reverse Diels-Alder reaction is another multicenter fragmentation often observed in mass spectrometry. Cyclic olefins in particular undergo this reaction to expel a neutral olefinic fragment (Equations 2.71 and 2.72). 

Like the McLafferty rearrangements, retro Diels-Alder reactions occur in many types of compounds with various double-bond systans. The aromatic ring of tetralin, for example, promotes the expulsion of ethylene from its molecular ion (Equation 2.73). 

In complex molecules containing several rings, this reaction may not be initially apparent due either to no loss of mass or to an unstable product that decomposes rapidly and therefore gives only a weak peak (Equation 2.74). Succeeding fragmentations, however, may show that the reaction did occur. 

The mass spectra of the isomeric bicyclic bromides CyHgBr are shown in Fignres 2.45 to 2.49. Match each spectrnm to one of the componnds. 

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Base peak in methylesters of cortoic acids. Fragment formed by McLafferty rearrangement. 

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